Static electricity is an objective existence of natural phenomena, resulting in a variety of ways, such as contact, friction, electrical induction, etc.. The characteristics of static electricity are the characteristics of long time accumulation, high voltage, low power, small current and short action time.
The body‘s own actions or contact with other objects, separation, friction or induction and other factors, can generate thousands of volts or even a million volts of static electricity. Static electricity in a number of areas caused serious harm. Friction electrification and human body static electricity is the two major hazards in the electronics industry, often cause the operation of electronic and electrical products is not stable, or even damage.
In the production process, the main measures of electrostatic protection are electrostatic leakage, dissipation, neutralization, humidification, shielding and grounding. The human body electrostatic protection system mainly has anti-static wrist strap, wrist strap, foot heel belt, work clothes, shoes, hats, gloves or fingerstall etc., with electrostatic discharge, and shielding function.
ESD protection is of great significance for high density, small size and complex electronic devices. This paper discusses the use of TVS diode to prevent ESD, minimum breakdown voltage and breakdown current, maximum reverse leakage current and the rated reverse turn off voltage and other parameters influence on circuit and selection criteria, and introduces some typical applications for portable consumer electronic devices, set-top boxes, personal computer and video line protection, USB protection and RJ-45 interface.
With the rapid development of mobile products, printers, PC, DVD, set-top boxes (STB) and other products, consumers are demanding more and more advanced performance. Semiconductor components increasingly tend to be small, high density and functional complexity, especially in the area of the main board area of the main board such as fashion consumer electronics and portable products are vulnerable to the impact of electrostatic discharge. Some complex semiconductor functional circuits with deep sub micron technology and very fine line width wiring are more sensitive to the influence of transient process, which will lead to more acute problems.
ESD protection principle
There are several technologies for the protection of the circuit, when selecting the circuit protection components, if the designer chooses improper protective devices will only provide the wrong concept of security. The selection of circuit protection elements shall be determined in accordance with the conditions of the wiring to be protected, the available circuit board space, and the electrical characteristics of the protected circuit. In addition, it is necessary to understand the protection characteristics of the knowledge element, one of the important factors to be considered is to clamp voltage device. The clamping voltage is across transient voltage suppressors in ESD device (TVS) on the voltage, it is protected by IC strain voltage.
Because the oxide layer is relatively thin in the IC circuit made by the advanced process technology, the gate oxide layer is more vulnerable to damage. This means that the clamp voltage is higher in voltage protection device IC strain produced higher, and increased the probability of failure.
A lot of protection elements are designed to absorb a large amount of energy, because the element structure or design reason also has resulted in the clamp voltage is very high. The rheostat clamping voltage is too high, they can not provide effective protection of esd. In addition, due to the high capacitance varistor they cannot provide protection to the high speed data lines. TVS diode is the key technology to solve this problem, it has become the key technology to protect portable electronic equipment.
TVS diode is a solid state device specifically designed to absorb ESD energy and protect the system from ESD damage. If used properly, the TVS diode will limit the voltage across the protected device just over the rated operating voltage, but it is far below the threshold voltage.
It is worth noting that, regardless of the choice of how the TVS device, they are very important in the layout of the circuit board. TVS layout before the wire length should be reduced to the minimum, because the fast (0.7ns) ESD pulse may have an additional voltage resulting in decreased TVS protection capabilities.
ESD protection typical circuit
We can often see the ESD protection circuit as shown below:
RJ-45 (10/100M Ethernet network)
RJ-45 interface is widely used in the network connection of the interface devices, the typical application is the 10/100M Ethernet network.
Video line protection
At present, the common video output ports are designed with D-SUB, DVI (28 lines), SCART (19 lines) and D-TERMINAL (mainly used in Japanese products). Video data line has a high rate of data transmission, data transmission rate up to 480Mbps, some video data transmission rate can reach more than 1G, thus to choose low capacitance TVS, it is usually a low capacitance diode and TVS diode series capacitor to reduce the whole line (less than 3pF), to achieve high-speed circuit demand. Shanghai Lei Mao electronics company ULC0524P has low good capacitance characteristics, can provide 4 ESD protection, and its successor SRDA05-6 can provide up to 6 ESD protection. SRDA05-X series products are suitable for all high speed communication line protection.
SIM card data line protection
SIM card data line protection has been the company‘s key products, and specifically for the design of such port set ESD (TVS) /EMI/RFI protection on a chip device, fully embodies the infinite integration of chip devices.
Button / switch
For the keys and switch circuit, the data rate of these circuits is very low, there is no special requirements for the device‘s capacitors, with ordinary TVS array can be competent. The above devices are used in the mobile phone ESD protection, in fact, is a 5V of anti-static, can meet the needs of static electricity.
Hardware design should pay attention to the problem
As foreign imports of electronic products are required by electrostatic testing, which requires us to design a product should be fully consider the ESD problem, in the design of hardware should pay attention to the following aspects of the problem:
First, static electricity will be through two channels in communication, one is air induction discharge, the other is conduction; air discharge is the main protective measures of induction shielding, mainly consider the structure and other aspects of the problem; the door is the main conduction absorption, as long as we can increase the ESD to protect the port before;
Secondly, as far as we increase the protection device ESD, try to stay away from the protected ports as far as possible;
At the time, in the design of PCB by ESD during the line, try to produce acute;
Finally, we should focus on the protection of the device to the ground into the problem, whether it is all included in the scope of protection, or in doing static reverse pulse test, can not achieve the level of the positive pulse.
In the choice of devices for different purposes, to avoid the device works in the design parameters near the limit, should also be based on the characteristics of the protected circuit device characteristics and can withstand ESD shock response speed is fast enough, the sensitivity is high enough, the protection device to effectively play the crucial role, another integrated device other functions should also be considered first.
From the electronic related article:
Power protection circuit (voltage regulator tube and transient suppression diode [TVS])
The use of transient suppression diodes 
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